The invention relates generally to blood chemistry monitoring utilizing enzyme-based blood analysis systems, such as blood glucose or cholesterol systems, and more specifically to portable blood chemistry monitoring meters for use with enzyme-based blood analysis systems.
Portable blood glucose monitoring meters were first made available for use in the late 1970's. Portable meters provided patients and health care providers with the means to improve insulin control by permitting them to determine blood glucose levels quickly and with reasonable accuracy, without the need for vein puncture and laboratory analysis. Since the introduction of such meters, improvements to date have produced portable meters offering greater convenience in smaller sizes with more features.
Portable blood glucose monitoring meters today typically utilize disposable test strips, similar to litmus paper, that have applied chemistries that either produce a color change or a change in electrical resistance when a drop of a patient's capillary blood is applied to the chemistries. In the case of test strips with chemistries that produce a color change, the strip becomes darker in proportion to the amount of blood glucose present in the blood. In such cases, the strip bearing the patient's blood is inserted into the meter and the color change in the chemistry on the strip is measured using an optical reflectance system within the meter. A microprocessor-based program within the meter then processes the color change measurement and generates a digital readout of the corresponding concentration, typically in milligrams per milliliter, of blood glucose in the patient's capillary blood. Such meters are commonly known as reflectance meters, and they are the most common type of portable blood glucose monitoring meter in use today.
In the case of test strips with applied chemistries that change in electrical resistance when a drop of a patient's capillary blood is applied to the chemistries, the change in electrical resistance is proportional to the concentration of blood glucose in the blood. In such cases, the strip is inserted into the meter and the change in electrical resistance is measured by the meter. A microprocessor-based program within the meter then processes the electrical resistance measurement and generates a digital readout of the corresponding concentration in milligrams per milliliter of blood glucose in the patient's capillary blood. Such meters are the least common type of portable blood glucose monitoring meters in use today.
In addition to generating digital readouts of blood glucose concentrations, another key function of the microprocessors found in all portable blood glucose monitoring meters in use today is the application of empirically derived correction factors to account for slight variations in the chemistries applied to test strips at the time of manufacture. Test strips are manufactured in batch lots. There is invariably slight lot-to-lot variations in the chemistries applied to test strips due to the complexities of the chemistries involved. It is therefore necessary to calibrate all portable blood glucose monitoring meters in use today to account for such lot-to-lot variations to assure consistent and accurate relationships between the color or resistance changes in such chemistries as measured by the meters and the corresponding blood glucose concentrations generated by the meter's microprocessor.
Lot-specific calibration factors for test strips are empirically derived by the test strip manufacturers. The lot specific calibration factors then accompany each package of test strips in a format that is suitable for use in a particular meter model. This has been accomplished to date in several different ways.
In one case, a bar code reader is provided in the meter. Lot-specific correction factors are provided with each package of test strips in a bar code format that can be read into the meter's microprocessor through the meter's bar code reader. Another method requires that a lot-specific code number be entered into the meter's microprocessor using keys located on the meter. The code number calls up a particular correction curve that has been programmed into the meter's microprocessor. Yet another method provides an electronic module with each package of test strips that is inserted into a receiving socket on the meter. The module houses an electronic memory element that contains the lot-specific correcting information for the meter's microprocessor.
Whatever the precise method utilized to enter the lot-specific correction factors into the meter's microprocessor, the microprocessor applies the lot-specific correction factors to the measurements taken by the meter of the blood glucose-induced changes in test strip chemistries, and then generates a corrected digital readout of the patient's capillary blood glucose concentration.
The drive to reduce the manufacturing costs of portable blood glucose monitoring meters led to a review of the foregoing methods of determining blood glucose concentrations. It was found that all portable blood glucose monitoring meters in use today have a common minimum manufacturing cost factor: the lower limit is determined by the cost of the microprocessor and the microprocessor-based electronic circuitry. Efforts to reduce the cost to patients of portable blood glucose monitoring meters are therefore impeded by the fixed costs of microprocessors and their related electronic circuitry.
The portable blood chemistry monitoring meters of the present invention employ a different and less expensive approach to enzyme-based blood analysis through measurements taken of the blood-induced changes in the chemistries of disposable test strips. The meters of the present invention provide a patient-operated devices that feature man-readable, replaceable, lot-specific calibration means to account for the lot-to-lot variation in test strip chemistries without the use of a microprocessor in one embodiment, and man-readable, individually calibrated, disposable test strips in yet another embodiment. The meters of the present invention therefore employ a more simple and less costly approach to enzyme-based blood analysis than is employed in any of the portable blood chemistry monitoring meters in use today.